Our long range goals are to study the interrelationships and possible interconversions of internal membrane systems with those of the plasma membrane during endocytosis in the soil ameba, Acanthamoeba castellani. We are using primarily a morphological approach that includes electron microscopy, freeze-fracture replication and cytochemistry. The present report describes the cytochemical localization of acid phosphatase activity which makes it possible to study the timing of lysosome-phagosome fusions. In a collaborative study, the fine structure of the bacterium Methanococcus vannielii was described with emphasis on the cell wall structure.